How are you checking clearance inside with it assembled? Snaking a bore scope in from the trans side?

I used a frame that I did cut in half ones to shorten a swingarm. And now used it to mount a swingarm and gearbox and see wat kind of room there was for the shaft to move around. This way I could have a good look.
So I did say "measured" but it was more "looked" at the clearances..

Have seen some bikes that had a lot of machining being done on the shaft and the inside of the swingarm just like Rucksta is explaining about. Done by The Boxertoko in The Netherlands.
For riding off road and not leaving the country to far I wouldn't mind the machined bits.
But I tried to build something with the weak parts like the u-joint original. So when necessary I can just order that part nr.and replace it.